Suppressor lymphokines may be important in the regulation of the immune response. We studied in vitro a murine suppressor lymphokine induced by Concanavalin A. This is a nonspecific suppressor which can suppress both a polyclonal blast transformation induced by lipopolysaccharide (LPS) and antibody synthesis induced by sheep red blood cells (SRBC). Initial steps in purification by sephadex gel filtration showed it to have a molecular weight of 36-72,000 daltons.